The phenolic resin molding compound is a material having an excellent balance of heat resistance, electrical properties, mechanical properties, dimensional stability, and the like. Therefore, they are utilized in a varieties of fields including electrical components. Generally, they are molded by injection molding. The phenolic resin molding compound loses its flowability when it is in the melted status, in which the phenolic resin molding compound is plasticized at 90° C. to 120° C. in the cylinder of the injection molding machine, since its viscosity increases due to the progress of the curing reaction of the resin. Thus, the thermal stability of the melted resin is reduced. Since the low stability of the melted molding compounds in the cylinder of the injection molding machine, the applicable condition is narrowly limited in a case where moldings are formed with an injection molding machine using the conventional phenolic resin molding compounds.
The viscosity of the generally used phenolic resin molding compounds are relatively high even in a state where they are plasticized at 90° C. to 120° C. in the cylinder. And, the viscosity increases further rapidly during its curing in the injection process where the phenolic resin molding compound is injected into the mold at 160° C. to 200° C. in order to cure it. Thus, the time span, in which the phenolic resin molding compound retains its flowability, is short. Therefore, in order to obtain high quality moldings, it is required to inject the material into the mold in a very short period of time at a very high pressure. However, a residual stress is generated in the moldings formed in the mold in this case. This leads to warp and deformation of the moldings when it is taken out from the mold and the stress is diffused during cooling process. In addition, it is very difficult to avoid burr formation, since a clearance is easily formed between molds when the resin is injected into the mold at a high pressure.
Conventionally, in order to solve the above-mentioned problem, molding is performed by reducing the melt viscosity of the molding compound to increase its flowability and injecting the molding compound at a low pressure. In this approach, the burr formation can be suppressed. However, this approach is difficult to apply to the practical usage since the molding cycle become longer due to slow curing.
On the other hand, in the injection molding using the phenolic resin molding compound, a reduced production yield problem occurs since dimension of the moldings can be easily varied and the dimension does not fit into the tolerance range. To compensate that, a precision processing is performed afterward. However, improving the production yield and reducing the post-injection precision processing are needed in order to reduce the production cost.
A technique to improve the thermal stability of the phenolic resin molding compound in the cylinder is disclosed in Patent Literature (PTL) 1. However, further improvement is needed in order to improve the dimensional precision.
On the other hand, a technique to improve the dimensional precision of the injected moldings by adding the talc or clay to the phenolic resin molding compound is disclosed in PTL2. However, further improvement is needed in order to improve its thermal stability in the cylinder.